It is known that the magnitude of a charged particle beam current in the accelerators designed for industrial application is a basic factor defining the radiation dose value within the accelerator irradiation field. When the material is treated by a charged particle beam, for example, by an electron beam, it acquires certain predetermined properties depending on the radiation dose. Unstability of the radiation dose results in deviation and spread of the properties of the irradiated material from the predetermined specifications. In order to provide the stability of the radiation dose it is very important to stabilize the electron beam current.
The magnitude of the electron beam current is adjusted generally by alteration of the cathode emission current varying either the cathode heater current or the intensity of the electric field around the cathode of the accelerating tube.
Known in the art is an electron beam current stabilizing device (Cf. an article by Akoulov V. V. et al. "Promishlennie uskoriteli serii "Elektron" dlja radiatsionnoi himii", preprint of NIIEFA No. D-0198, Leningrad, 1974, p.II), comprising a ferroresonant voltage regulator connected to a secondary winding of an accelerator high-voltage transformer, designed to supply the accelerating tube cathode heater, and an adjustable autotransformer connected to the output of the ferroresonant regulator. Connected to the output of the autotransformer is a primary winding of the cathode heater transformer, the autotransformer being regulated by means of a reversible electric motor the shaft of which is coupled through an insulating bar with a sliding contact of the autotransformer. The reversible motor is actuated by an operator controlling the magnitude of the electron beam current.
The aforementioned device provides considerably low beam current stability because of, in the first place, low stability of ferroresonant voltage regulators when alteration of the voltage in a supply network occurs, in the second place, the ambiquous relation between the cathode heater voltage and the cathode temperature (the resistance of the heater circuit and herewith the heater current may vary in the course of operation resulting in the alteration of the cathode temperature which brings, in its turn, again to the alteration of the heater circuit resistance etc.) whereas it is known that the cathode emission current depends exactly on the cathode temperature and, in the third place, the loss of emissive properties by the activated cathode due to the aging of said cathode.
Besides, the aforementioned device fails to provide automatic adjustment of the beam current and therefore a considerably large period of time may pass from the moment of alteration of the beam current to the moment of effecting the control, during which the radiation dose will not correspond to a rated value thus leaving a part of the treated material devoid of the required property.
Known in the art is an electron beam current stabilizing device (C.f. Japanese Pat. No. 34514, published in 1974), comprising a thyristor current stabilizer inserted in the cathode heater circuit. The beam current is adjusted in this device, similar to the aforementioned one, by an operator actuating the current stabilizer through insulating bar coupled with the shaft of the reversible electric motor. The device according to this patent provides better stability of the beam current since the heater current and not the heater voltage define the temperature of the cathode and consequenty the beam current. However, the stability of the beam current is still not sufficient due to participation of a man in the process of control.
Also known in the art is an electron beam current stabilizing device (Cf. U.S. Pat. No. 3,293,483 published in 1966), comprising a photosensitive element connected to a cathode of an accelerating tube to adjust the beam current, and a light source controlling this photosensitive element. The photosensitive element is made as a photoresistor. The cathode of the accelerating tube is connected through a secondary winding of the heater transformer and through the photoresistor with a negative output of the acceleration voltage source. The negative output of the acceleration voltage source is also connected to an accelerating tube modulator disposed near the cathode. The primary winding of the heater transformer is connected generally to one of the secondary windings of the high-voltage transformer of the acceleration voltage source.
Means are provided in the device for regulation of the intensity, or brightness, or the spectral composition of the light emanating from the light source, which allow the resistance of the photoresistor to be set such that the potential difference between the cathode and the modulator of the accelerating tube at a given particular value of the acceleration voltage corresponds to the predetermined beam current.
The required beam current value is maintained automatically owing to the fact that when the beam current is deviated from the required value, voltage drops at the photoresistor and therewith the potential difference between the cathode and the modulator is altered, said alteration of the potential difference being characterized by the reduction of said beam current deviation caused by said alteration.
The device according to this patent also fails to provide the sufficient accuracy of stabilization of the electron beam current, which is attributed first of all to the low stability of the intensity and spectral composition of the light radiation as well as to the effect of various interferences upon the transmitted analogue light signal. Unstability of the light radiation results in unstability of the resistance of the photoresistor. Since the light source is not inserted in the control circuit formed by the photoresistor, the cathode of the accelerating tube and the modulator, the regulation error will be proportional to the variation in the parameters of the light source. The accuracy of the beam current stabilization is reduced also because of a considerable temperature unstability of the resistance of the photoresistor approaching about 0.5-3% per a degree whereas the temperature of the accelerator may vary in the course of its operation by as much as 30-40 degrees.
Furthermore the accuracy of stabilization in this device depends on the beam current magnitude and, namely, decreases with the increase in the beam current magnitude, which is explained by the fact that the greater is the portion of the acceleration voltage that drops at the photoresistor, the higher is the accuracy of stabilization. When it is necessary to increase the beam current the operator should decrease the resistance of the photoresistor through changing the light influence, thus decreasing the portion of the acceleration voltage of the photoresistor, the stability of the beam current being consequently lowered.
What is more, the increase in the beam current is accompanied also by additional temperature unstability of the beam current attributed to the heat release at the photoresistor when the beam current flows across it. Thus, for example, to reach the accuracy of stabilization of the order of about several percent, the voltage drop at the photoresistor should be about 2-5% of the acceleration voltage. It does not bring to considerable heating of the photoresistor when the light beam current is equal, for example, to 100 microamperes, since the power released at the photoresistor does not exceed several watts, while the power of about several kilowatts will be released on the photoresistor when the beam current is equal, for example, to 100 milliamperes and the acceleration voltage is equal, for example, to 500 kilovolts. Since heat removal from the high potential area of the accelerator presents difficulties, the photoresistor will be strongly heated whereby its resistance will considerably vary resulting in the alteration of the voltage drop at the photoresistor and therewith the beam current, i.e. in unstability of the beam current.
The device according to the U.S. Pat. No. 3,293,483 cannot be also used for stabilization of heavy beam currents in modern high-power accelerators designed for industrial application because all photoresistors known at present are capable to pass the currents not exceeding several milliamperes, the permissible voltages not exceeding the tens of volts. In modern accelerators maximum beam currents reach hundreds of milliamperes and the potential of the modulator with respect to the cathode may reach several kilovolts. That is why in order to stabilize such beam currents in the aforesaid device at least several hundreds of photoresistors would be required, connected in parallel and in series which, being of considerable dimensions, would occupy a large area under the high potential where the space is extremely limited.
Besides, the device according to the U.S. Pat. No. 3,293,483 is characterized by considerable inertia because of photoresistor inertia, so that the time from the moment of setting the required resistance value of the photoresistor to the moment when the beam current reaches the required value may be long enough, whereby a part of the material being treated is irradiated by a reduced dose and thus becomes rejected.